This invention relates to oriented shaped articles. More specifically, this invention relates to a process for forming oriented shaped articles comprising a block copolymer comprising 75% to 90% by weight of polyethylene terephthalate blocks and 10% to 25% by weight of polyethylene isophthalate blocks.
Polyethylene terephthalate (PET) resin is used in the packaging industry, including the production of containers for containing a wide variety of carbonated and non-carbonated fluids. The barrier properties of the container are often critical, particularly in the carbonated beverage industry, where bottlers are concerned about the rate at which the container allows carbon dioxide to escape from the container. If carbon dioxide escapes too rapidly, the beverage will go flat on the shelf before it can be sold and used by the consumer.
Among the improvements which have been disclosed in recent years is the incorporation of varying amounts of polyethylene isophthalate (PEI) into polyethylene terephthalate resins. The resulting PEI/PET resins have been found to have improved barrier properties over that of PET containers and, thus, have led to increases in the shelf life of many products. For oriented shaped articles requiring a longer shelf life, PEI has been used as a barrier layer in a multi-layer container or as a blend with PET in single-walled containers.
U.S. Pat. No. 4,403,090 to Smith discloses a method for making block copolyesters by separately forming isophthalic and non-isophthalic polyesters, melt blending the polyesters, and then polymerizing the melt blend in the solid state. Though detailed solid state polymerization conditions are provided, no specific methods beyond the foregoing are disclosed for making and orienting the block copolymers.
U.S. Pat. No. 4,643,925 to Smith et al. discloses a high molecular weight polyester resin prepared by solid state polymerizing a melt blend of PET and PEI homopolymers. Prepolymers of the component polymers having an inherent viscosity of at least 0.3 are first melt blended, solidified into pellets or chips, crystallized, and then solid state polymerized at about 5xc2x0 C. to 20xc2x0 C. below the sticking temperature of the pellets.
U.S. Pat. No. 6,150,454 to Wu et al. discloses a copolyester composition made from a random copolymer of isophthalic and terephthalic acids, a nucleating agent, and a chain-branching agent. It is stated that the chain-branching agent is added to reduce the natural stretch ratio of the copolymer resins to about the stretch ratio levels of commercially available PET resins. The copolymers in Wu et al. are produced by combining the acids, glycols, branching agents, and nucleating agents in the melt and polymerizing to form the branched, random copolymers of patentees invention. Wu et al.""s disclosure is limited to up to 10% of PIA comonomer. It is well known in the art that the mechanical integrity of containers made of random TA/IA copolymers deteriorate rapidly with increasing amounts of the IA moiety above 10%.
The Japan Patent Application Publications H10-279784 and H11-322 968 to Kawano disclose making rigid containers having improved barrier properties using block copolymers formed from PEI and PET moieties. Kawano explicitly teaches forming containers by blow molding employing the same average planar stretch ratio for PET homopolymer resins and the block PEI/PET copolyester resins of Kawano""s invention. The planar stretch ratios taught by Kawano are those in widespread commercial use for PET homopolymer.
There is a need for additional methods of forming oriented shaped articles that have low permeability to gas molecules.
The invention provides a process for producing an oriented shaped article. The process comprises the steps of (a) forming a shaped article comprising a block copolymer comprising 75% to 90% by weight of polyethylene terephthalate blocks and 10% to 25% by weight of polyethylene isophthalate blocks and (b) biaxially stretching the shaped article to an average planar stretch ratio of 10 to 30.
In one embodiment, the polyethylene terephthalate used in the process of the invention is a branched polyethylene terephthalate. In another embodiment, the block copolymer has a randomness factor of 0.3-0.7, and in another embodiment, the randomness factor of the block copolymer is 0.4 to 0.6. In yet another embodiment, the block copolymer has an intrinsic viscosity of at least 0.6 dl/g. In another embodiment, the block copolymer has an intrinsic viscosity of at least 0.7 dl/g. In yet another embodiment of the invention, the copolymer comprises 15% to 20% by weight of polyethylene isophthalate blocks.
The oriented shaped article, in one embodiment, is a film. In another embodiment, the oriented shaped article is a container. In yet another embodiment, the oriented shaped article is a bottle suitable for holding carbonated liquids.
In one embodiment, the step of biaxially stretching comprises blow molding. In another embodiment, the process further comprises the step of (c) heat setting the article.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention.